The present invention relates to a gate valve capable of shearing a wire line or small pipe extending through its bore. Gate valves have been used for shearing through wire lines and small tubular members but difficulties have been encountered after a shearing has taken place with prior shearing gate valves. Often the problem is that the valve which has accomplished the shearing no longer has a satisfactory seal. An additional problem is that prior shearing gate valves have also damaged the shearing edges during their initial shearing operation and had to be repaired prior to being placed in service thereafter.
U.S. Pat. Nos. 4,519,575 and 4,612,983 disclose typical prior art gate valves adapted for shearing a wire line extending through the valve passages and the gate. U.S. Pat. No. 4,519,575 discloses a gate valve for shearing a wire line on closing and the gate valve includes a special actuator which adds closing force during the last or shearing portion of the stroke. U.S. Pat. No. 4,612,983 discloses a gate valve having a pair of gates in which the lower gate closes and shears the wire line and then the upper gate closes and seals against the upper valve seat.
U.S. Pat. No. 4,281,819 discloses a balanced stem gate valve in which the sealing surfaces are hard, friction resistant sealing surfaces and suggests valve seats of metal or non-metal seat material concentrically disposed interiorly of seat retainers and sealed with sealing rings. Back-up seat rings are provided and are of a hard material such as tungsten carbide.
U.S. Pat. Nos. 4,081,027 and 4,341,264 disclose ram type blowout preventers having shearing type of rams in which the leading edge of the shearing blade of each ram is provided with a hardened shearing surface. In U.S. Pat. No. 4,081,027 it is suggested that the rams are hardfaced with a layer of nickel-based alloys, cobalt-based alloy and tungsten-based alloys to allow the shearing of a drill pipe. U.S. Pat. No. 4,341,264 suggest the use of a hardened material insert on the leading edges of the shear blades so that it is replaceable and the ram blade can be repaired after any damage during shearing.
U.S. Pat. No. 4,425,699 discloses the method of connecting a mechanical seal ring of hard metal or ceramic to a supporting ring made of aluminum or stainless steel which includes the pressing of the two rings against each other with such force that the supporting ring yields to provide a permanent engagement between the rings.
U.S. Pat. No. 4,337,920 discloses a valve or choke for use in high temperature and extremely abrasive service which includes the use of a sleeve of abrasion resistant material, such as cemented tungsten carbide, silicon carbide, titanium diboride, ceramic material or other material. The sleeve is supported by springs to prevent its damage from vibrations and to insulate it from the effects of differential expansion.